Exhaust system with a modified lean NOx trap

ABSTRACT

An exhaust system for treating an exhaust gas from an internal combustion engine is disclosed. The system comprises a modified lean NOx trap (LNT), a urea injection system, and an ammonia-selective catalytic reduction catalyst. The modified LNT comprises a first layer and a second layer. The first layer comprises a NOx adsorbent component and one or more platinum group metals. The second layer comprises a diesel oxidation catalyst zone and an NO oxidation zone. The diesel oxidation catalyst zone comprises a platinum group metal, a zeolite, and optionally an alkaline earth metal. The NO oxidation zone comprises a platinum group metal and a carrier. The modified LNT stores NOx at temperatures below about 200° C. and releases at temperatures above about 200° C. The modified LNT and a method of using the modified LNT are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.14/820,581, allowed on Jul. 26, 2017, which claims priority benefit toU.S. Provisional Patent Application No. 62/036,184 filed on Aug. 12,2014, both of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an exhaust system for treating an exhaust gasfrom an internal combustion engine, and a method for treating exhaustgas from internal combustion engines.

BACKGROUND OF THE INVENTION

Internal combustion engines produce exhaust gases containing a varietyof pollutants, including nitrogen oxides (“NO_(x)”), carbon monoxide,and uncombusted hydrocarbons, which are the subject of governmentallegislation. Emission control systems are widely utilized to reduce theamount of these pollutants emitted to atmosphere, and typically achievevery high efficiencies once they reach their operating temperature(typically, 200° C. and higher). However, these systems are relativelyinefficient below their operating temperature (the “cold start” period).

For instance, current urea based selective catalytic reduction (SCR)applications implemented for meeting Euro 6b emissions require that thetemperature at the urea dosing position be above about 180° C. beforeurea can be dosed and used to convert NO_(x). NO_(x) conversion below180° C. is difficult to address using the current systems, and futureEuropean and US legislation will stress the low temperature NO_(x)storage and conversion. Currently this is achieved by heating strategiesbut this has a detrimental effect of CO₂ emissions.

As even more stringent national and regional legislation lowers theamount of pollutants that can be emitted from diesel or gasolineengines, reducing emissions during the cold start period is becoming amajor challenge. Thus, methods for reducing the level of NO_(x) emittedduring cold start condition continue to be explored.

For instance, PCT Intl. Appl. WO 2008/047170 discloses a system whereinNO_(x) from a lean exhaust gas is adsorbed at temperatures below 200° C.and is subsequently thermally desorbed above 200° C. The NO_(x)adsorbent is taught to consist of palladium and a cerium oxide or amixed oxide or composite oxide containing cerium and at least one othertransition metal.

U.S. Appl. Pub. No. 2011/0005200 teaches a catalyst system thatsimultaneously removes ammonia and enhances net NO_(x) conversion byplacing an ammonia-selective catalytic reduction (“NH₃-SCR”) catalystformulation downstream of a lean NO_(x) trap. The NH₃-SCR catalyst istaught to adsorb the ammonia that is generated during the rich pulses inthe lean NO_(x) trap. The stored ammonia then reacts with the NO_(x)emitted from the upstream lean NO_(x) trap, which increases NO_(x)conversion rate while depleting the stored ammonia.

PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purificationsystem which includes a NO_(x) storage catalyst arranged upstream of anSCR catalyst. The NO_(x) storage catalyst includes at least one alkali,alkaline earth, or rare earth metal which is coated or activated with atleast one platinum group metal (Pt, Pd, Rh, or Ir). A particularlypreferred NO_(x) storage catalyst is taught to include cerium oxidecoated with platinum and additionally platinum as an oxidizing catalyston a support based on aluminum oxide. EP 1027919 discloses a NO_(x)adsorbent material that comprises a porous support material, such asalumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1wt % precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina isexemplified.

As with any automotive system and process, it is desirable to attainstill further improvements in exhaust gas treatment systems,particularly under cold start conditions. We have discovered a systemthat can reduce NO_(x) emissions during the cold start period, whilemaintaining good CO oxidation activity and showing resistance todeactivation by sulfation.

SUMMARY OF THE INVENTION

The invention is an exhaust system for treating an exhaust gas from aninternal combustion engine. The system comprises a modified lean NO_(x)trap (LNT), a urea injection system, and an ammonia-selective catalyticreduction (NH₃-SCR) catalyst. The modified LNT comprises a first layerand a second layer. The first layer comprises a NO_(x) adsorbentcomponent and one or more platinum group metals. The second layercomprises a diesel oxidation catalyst zone and a NO oxidation zone. Thediesel oxidation catalyst zone comprises a platinum group metal, azeolite, and optionally an alkaline earth metal. The NO oxidation zonecomprises a platinum group metal and a carrier. The modified LNT storesNO_(x) at temperatures below about 200° C. and releases at temperaturesabove about 200° C. The invention also includes the modified LNT and amethod of using the modified LNT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the cumulative NO_(x) from engine testing with the modifiedLNT.

FIG. 2 shows the improved concentration of NO₂/NO_(x) ratio for a LNTthat comprises a diesel oxidation catalyst zone and a NO oxidation zoneover a first layer that comprises a NO_(x) adsorbent.

DETAILED DESCRIPTION OF THE INVENTION

The invention is an exhaust system for treating an exhaust gas from aninternal combustion engine. The system comprises a modified lean NO_(x)trap (LNT). Lean NO_(x) traps are well known in the art. Lean NO_(x)trap are typically designed to adsorb NO_(x) under lean exhaustconditions, release the adsorbed NO_(x) under rich conditions, andreduce the released NO_(x) to form N₂.

LNTs typically include a NO_(x)-storage component, an oxidationcomponent, and a reduction component. The NO_(x)-storage componentpreferably comprises alkaline earth metals (such as barium, calcium,strontium, and magnesium), alkali metals (such as potassium, sodium,lithium, and cesium), rare earth metals (such as lanthanum, yttrium,praseodymium and neodymium), or combinations thereof. These metals aretypically found in the form of oxides. Typically, platinum is includedto perform the oxidation function and rhodium is included to perform thereduction function. These components are contained on one or moresupports.

The oxidation/reduction catalyst and the NO_(x)-storage component arepreferably loaded on a support material such as an inorganic oxide toform an LNT for use in the exhaust system.

The modified LNT of the invention is designed to have a differentfunction than known LNTs, in that they are preferably designed to storeNO_(x) at temperatures below about 200° C. and release the stored NO_(x)at temperatures above about 200° C. The release of stored NO_(x) may bebrought about thermally or may also be brought about by a rich purge.

The modified LNT comprises a first layer and a second layer. The firstlayer comprises a NO_(x) adsorbent component and one or more platinumgroup metals. The NO_(x) adsorbent component preferably comprises analkaline earth metal, an alkali metal, a rare earth metal, and mixturesthereof. The alkaline earth metal is preferably barium, calcium,strontium, or magnesium. The alkali metal is preferably potassium,sodium, lithium, or cesium. The rare earth metal is preferablylanthanum, yttrium, praseodymium, or neodymium. Most preferably, theNO_(x) adsorbent component comprises barium.

If utilized, the alkaline earth metal, alkali metal, rare earth metal,or mixtures thereof may preferably be supported on inorganic oxide. Theinorganic oxide material is preferably a ceria-containing material or amagnesia-alumina. The ceria-containing material is preferably ceria,ceria-zirconia, ceria-zirconia-alumina, or mixtures thereof. Morepreferably, the ceria-containing material is ceria, and in particular,particulate ceria. The magnesia-alumina is preferably a spinel, amagnesia-alumina mixed metal oxide, a hydrotalcite or hydrotalcite-likematerial, and combinations of two or more thereof. More preferably, themagnesia-alumina support is a spinel.

The alkaline earth metal, an alkali metal, or a rare earth metalcomponent may be loaded onto the inorganic oxide material by any knownmeans, the manner of addition is not considered to be particularlycritical. For example, a barium compound (such as barium acetate) may beadded to a ceria-containing material by impregnation, adsorption,ion-exchange, incipient wetness, precipitation, or the like. Preferably,the first layer comprises at least 2.5 weight percent barium.

The platinum group metal is preferably platinum, palladium, rhodium, ormixtures. Platinum and palladium are particularly preferred.

Preferably, the first layer also comprises a support. The support ispreferably an oxide of Groups 2, 3, 4, 5, 13 and 14 elements. Mostpreferably, the support is an alumina, silica, titanic, zirconia,magnesia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide, amixed oxide or composite oxide of any two or more thereof, and mixturesthereof. Preferred supports preferably have surface areas in the range10 to 1500 m²/g, pore volumes in the range 0.1 to 4 mL/g, and porediameters from about 10 to 1000 Angstroms. High surface area supportshaving a surface area greater than 80 m²/g are particularly preferred.

The second layer comprises a diesel oxidation catalyst zone and an NOoxidation zone. The diesel oxidation catalyst zone comprises a platinumgroup metal, a zeolite, and optionally an alkaline earth metal. Theoptional alkaline earth metal is preferably magnesium, calcium,strontium, or barium; more preferably, barium. The platinum group metalpreferably comprises platinum and palladium. Preferably, the Pd:Pt ratioin the diesel oxidation catalyst zone ranges from 0.25 to 1.

The zeolite may be any natural or a synthetic zeolite, and is preferablycomposed of aluminum, silicon, and/or phosphorus. The zeolites typicallyhave a three-dimensional arrangement of SiO₄, AlO₄, and/or PO₄ that arejoined by the sharing of oxygen atoms, but may also be two-dimensionalstructures as well. The zeolite frameworks are typically anionic, whichare counterbalanced by charge compensating cations, typically alkali andalkaline earth elements (e.g., Na, K, Mg, Ca, Sr, and Ba), ammoniumions, and also protons.

Preferably, the zeolite is selected from an aluminosilicate zeolite, ametal-substituted aluminosilicate zeolite, an aluminophosphate zeolite,a metal-substituted aluminophosphate zeolite, a silicoaluminophosphatezeolite, or a metal-substituted silicoaluminophosphate zeolite. Zeoliteshaving a Framework Type of ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD,ATT, CDO, CHA, DDR, OFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW,LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO,TSC, UEI, UFI, VNI, YUG, ZON, MFI, FER, MWW, EUO, CON, BEA, FAU, MOR,and EMT, as well as mixtures or intergrowths of any two or more, areparticularly preferred. Most preferably, the zeolite has a FrameworkType of an AEI, a CHA, a LEV, a BEA (e.g., beta zeolite), a FAU (e.g.,zeolite Y), or MFI (e.g., ZSM-5).

The diesel oxidation catalyst may also preferably comprise manganese.

The diesel oxidation catalyst zone preferably also comprises aninorganic oxide support. The inorganic oxide support preferably includesoxides of Groups 2, 3, 4, 5, 13 and 14 elements. Most preferably, thesupport is an alumina or silica-doped alumina support.

The NO oxidation zone comprises a platinum group metal and a carrier.The platinum group metal preferably comprises platinum and palladium.Preferably, the Pd:Pt ratio in the NO oxidation zone ranges from 0 to0.25. The carrier is preferably alumina, silica, a ceria-containingmaterial, titania, zirconia, magnesia, niobia, tantalum oxide,molybdenum oxide, tungsten oxide, a mixed oxide or composite oxide ofany two or more thereof (e.g. silica-alumina, magnesia-alumina), andmixtures thereof. The ceria-containing material is preferably ceria,ceria-zirconia, ceria-zirconia-alumina, or mixtures thereof; and morepreferably, the ceria-containing material is ceria, and in particular,particulate ceria. Mixtures of carriers such as alumina and ceria areparticularly preferred. The NO oxidation zone may also preferablycomprise manganese. The NO oxidation zone may contain an alkali metal oralkaline earth metal such as barium, but may also be substantially freeof an alkali metal or alkaline earth metal component. By “substantiallyfree”, it is meant that no alkali metal or alkaline earth metal isdeliberately added to the NO oxidation zone. Preferably, “substantiallyfree” means that the NO oxidation zone contains less than 0.1 weightpercent alkali metal or alkaline earth metal, more preferably less than0.05 weight percent alkali metal or alkaline earth metal, and mostpreferably no alkali metal or alkaline earth metal.

The modified LNT preferably includes a substrate. The substrate ispreferably a ceramic substrate or a metallic substrate. The ceramicsubstrate may be made of any suitable refractory material, e.g.,alumina, silica, titania, ceria, zirconia, magnesia, zeolites, siliconnitride, silicon carbide, zirconium silicates, magnesium silicates,aluminosilicates and metallo aluminosilicates (such as cordierite andspodumene), or a mixture or mixed oxide of any two or more thereof.Cordierite, a magnesium aluminosilicate, and silicon carbide areparticularly preferred.

The metallic substrate may be made of any suitable metal, and inparticular heat-resistant metals and metal alloys such as titanium andstainless steel as well as ferritic alloys containing iron, nickel,chromium, and/or aluminum in addition to other trace metals.

The substrate is preferably a flow-through substrate or a filtersubstrate. Most preferably, the substrate is a flow-through substrate.In particular, the flow-through substrate is a flow-through monolithpreferably having a honeycomb structure with many small, parallelthin-walled channels running axially through the substrate and extendingthroughout the substrate. The channel cross-section of the substrate maybe any shape, but is preferably square, sinusoidal, triangular,rectangular, hexagonal, trapezoidal, circular, or oval.

When added to the substrate, the layers of the modified NO_(x) trap maybe arranged on the substrate in any order, but preferably the firstlayer is disposed on the substrate, and the second layer is disposed onthe first layer. The diesel oxidation catalyst zone of the second layeris preferably arranged upstream of the NO oxidation zone so that theexhaust gas first contacts the diesel oxidation catalyst zone prior tocontacting the NO oxidation zone.

The modified NO_(x) trap of the present invention may be prepared byprocesses well known in the prior art. Preferably, the NO_(x) trap isprepared by depositing the two layers on the substrate using washcoatprocedures.

Preferably, the entire length of the substrate is coated with the firstlayer slurry so that a washcoat of the first layer covers the entiresurface of the substrate. A portion of the length of the substrate fromthe front end is coated with the diesel oxidation catalyst zone, whilethe remainder of the substrate length is coated with the NO oxidationzone.

The modified LNT of the invention stores NO_(x) at temperatures belowabout 200° C. and releases the stored NO_(x) at temperatures above about200° C.

The exhaust system of the invention also comprises an ammonia-selectivecatalytic reduction (NH₃-SCR) catalyst. The NH₃-SCR catalyst maycomprise any known NH₃-SCR catalysts, which are well-known in the art. ANH₃-SCR catalyst is a catalyst that reduces NO_(x) to N₂ by reactionwith nitrogen compounds (such as ammonia or urea).

Preferably, the NH₃-SCR catalyst is comprised of a vanadia-titaniacatalyst, a vanadia-tungsta-titania catalyst, or a metal/zeolite. Themetal/zeolite catalyst comprises a metal and a zeolite. Preferred metalsinclude iron and copper.

The zeolite may be any natural or a synthetic zeolite, and is preferablycomposed of aluminum, silicon, and/or phosphorus. The zeolites typicallyhave a three-dimensional arrangement of SiO₄, AlO₄, and/or PO₄ that arejoined by the sharing of oxygen atoms, but may also be two-dimensionalstructures as well. The zeolite frameworks are typically anionic, whichare counterbalanced by charge compensating cations, typically alkali andalkaline earth elements (e.g., Na, K, Mg, Ca, Sr, and Ba), ammoniumions, and also protons.

Preferably, the zeolite is selected from an aluminosilicate zeolite, ametal-substituted aluminosilicate zeolite, an aluminophosphate zeolite,a metal-substituted aluminophosphate zeolite, a silicoaluminophosphatezeolite, or a metal-substituted silicoaluminophosphate zeolite. Zeoliteshaving a Framework Type of ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD,ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW,LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO,TSC, UEI, UFI, VNI, YUG, ZON, MFI, FER, MWW, EUO, CON, BEA, FAU, MOR,and EMT, as well as mixtures or intergrowths of any two or more, areparticularly preferred. Most preferably, the zeolite has a FrameworkType of an AEI, a CHA, a LEV, a BEA (e.g., beta zeolite), or a FER(e.g., ferrierite).

The NH₃-SCR catalyst is preferably coated on a ceramic or a metallicsubstrate, as described above. The substrate is typically designed toprovide a number of channels through which vehicle exhaust passes, andthe surface of the channels will be preferably be coated with theNH₃-SCR catalyst.

The substrate for the NH₃-SCR catalyst may be a filter substrate or aflow-through substrate. Preferably, the NH₃-SCR catalyst is coated ontoa filter, which is known as an ammonia-selective catalytic reductionfilter (NH₃-SCRF). SCRFs are single-substrate devices that combine thefunctionality of an NH₃-SCR and particulate filter. They are used toreduce NO_(x) and particulate emissions from internal combustionengines.

The system of the invention further comprises a urea injection system.The urea injection system preferably comprises a urea injector thatinjects urea into the exhaust gas stream upstream of the NH₃-SCRcatalyst and downstream of the modified LNT. The urea injection systemwill preferably consist of a nozzle to produce well defined droplets ofurea solution. The droplet size is preferably less than 500 microns toallow rapid evaporation and urea decomposition. The injector pressureand pump rate will be such to allow effective mixing in the exhaust gasstream.

The urea injection system will also preferably consist of a urea tank,transfer lines and possibly a heating system to avoid freezing of theurea solution.

Preferably, the urea injection system injects urea at temperatures aboveabout 180° C.

The invention also includes a method for treating an exhaust gas from aninternal combustion engine. The method comprises passing the exhaust gasover the modified LNT described above. The modified LNT removes oxidesof nitrogen (NO_(x)) from the exhaust gas at temperatures below about200° C., and releases the NO_(x) at temperatures above about 200° C. Attemperatures above about 180° C., urea is injected into the exhaust gasdownstream of the modified LNT, and the exhaust gas containing releasedNO_(x) from the modified LNT and urea is passed over a NH₃-SCR catalyst.The released NO_(x) is converted to nitrogen by the reaction of ammonia(generated from urea) with NO_(x) over the NH₃-SCR catalyst. Thereleased NO_(x) is the NO_(x) that is stored on the modified LNT at lowtemperatures and is then released at the higher temperatures, and alsoincludes NO_(x) that is passes over the NH₃-SCR NH₃-SCR catalyst withoutbeing stored.

Preferably, the modified LNT is periodically subjected to a richdesulfation step. The presence of sulfur compounds in fuel may bedetrimental to the modified LNT since the oxidation of sulfur compoundsleads to sulfur oxides in the exhaust gas. In the LNT, sulfur dioxidecan be oxidized to sulfur trioxide over the platinum group metals andform surface sulfates on the LNT surface (e.g., barium oxide or bariumcarbonate reacts with sulfur trioxide to form barium sulfate). Thesesulfates are more stable than the nitrates and require highertemperatures (>500° C.) to desulfate. If the modified LNT has a lowbarium content, a lower desulfation temperature may be useful.

In rich desulfation, the modified LNT is typically subjected to atemperature above about 500° C. in rich air:fuel ratio environment toaccomplish sulfur removal. The desulfation is preferably performed byincreasing exhaust temperatures through a post-injection of fuel.Desulfation strategies may include a single, continuous rich period, ora series of short rich air-to-fuel ratio pulses.

The following examples merely illustrate the invention. Those skilled inthe art will recognize many variations that are within the spirit of theinvention and scope of the claims.

Example 1: Preparation of Modified LNTs

Modified LNT 1A

A 400 cell per square inch (cpsi) flow-through cordierite substratemonolith is coated with a NO_(x) absorber catalyst formulationcomprising two layers. The lower layer washcoat comprises Pt, Pd, 28% ofa Ce/magnesium-aluminate spinel and 66% ceria (95% of the total cerialoading includes particulate ceria containing 7% Ba). The washcoat iscoated on the virgin substrate monolith using the method disclosed in WO99/47260, followed by drying for 30 minutes in a forced air drier at100° C. and then firing at 500° C. for 2 hours.

A second slurry is prepared consisting of alumina slurried and milled toa d₉₀<20 micron followed by the addition of appropriate amounts ofsoluble platinum and palladium salts and particulate ceria to give 50%alumina and 50% particulate ceria. The second slurry is applied to thecalcined lower layer via outlet channels. The part is dried and calcinedat 500° C.

A third slurry is prepared consisting of a silica-doped alumina powderslurried in water and milled to a d₉₀<20 micron. Barium acetate is addedto the slurry followed by appropriate amounts of soluble platinum andpalladium salts. The slurry is then stirred for 30 minutes to homogenizebefore addition of beta-zeolite to give 81% doped silica alumina and 19%beta zeolite. The third slurry is applied to the calcined lower layervia the inlet channels. The part is dried and calcined at 500° C. togive a total PGM loading of 89 g/ft³ Pt and 30 g/ft³ Pd.

Comparative LNT 2

A 400 cells per square inch (cpsi) flow-through cordierite substratemonolith is coated with a NO_(x) absorber catalyst formulationcomprising a single layer that comprises Pt, Pd, aCe/magnesium-aluminate spinel, a Ba coated particulate ceria, andcontains 33% of the Ce/magnesium-aluminate spinel, 61% of ceria (93% ofthe total ceria loading includes particulate ceria containing 7% Ba),and 113 g/ft³ of Pt and Pd. The washcoat is coated on the virginsubstrate monolith using the method disclosed in WO 99/47260, followedby drying for 30 minutes in a forced air drier at 100° C. and thenfiring at 500° C. for 2 hours.

Example 2: Engine Testing

Modified LNT 1A and Comparative LNT 2 (1.4 L catalyst volume) arehydrothermally aged at 800° C. for 5 hours. Each LNT is pre-conditionedon a 1.6 liter engine employing low pressure exhaust gas recirculation(EGR), by running 4 NEDC cycles with a 5 s rich purge at lambda 0.95 onthe 100 kph cruise. Evaluation is then carried out over the NEDC drivecycles on a 2.2 liter engine. No rich purging is employed during theevaluation.

The results show that the modified LNT 1A stores about 0.5 g NO_(x) upto about 200° C., followed by near complete thermal release of thestored NO_(x) from 200 to 300° C., showing that the modified LNT of theinvention are capable of use with a NH₃-SCR system. FIG. 2 shows thatthe modified LNT 1A design increases post LNT NO₂/NOx ratio to 20-40%compared to 5-20% for the comparative LNT 2. Table 1 describes thetailpipe CO emissions with the modified LNT 1A showing much greater COconversion and greatly improved stability to oxidative deactivation over4 NEDC without rich purging. Table 2 describes the tailpipe HC emissionswith the modified LNT 1A showing much greater HC conversion.

TABLE 1 Tailpipe CO emissions CO tailpipe emissions (g) ModifiedComparative Test Run LNT 1A LNT 2 NEDC # 1 after 2.40 3.73 richpre-conditon NEDC # 2 3.40 5.50 NEDC # 3 3.57 6.51 NEDC # 4 3.66 6.66

TABLE 2 Tailpipe HC emissions HC tailpipe emissions (g) ModifiedComparative Test Run LNT 1A LNT 2 NEDC # 1 after 0.44 0.63 richpre-conditon NEDC # 2 0.46 0.66 NEDC # 3 0.47 0.74 NEDC # 4 0.47 0.74

We claim:
 1. An exhaust system for treating an exhaust gas from aninternal combustion engine, comprising a modified lean NO_(x) trap(LNT), a urea injection system, and an ammonia-selective catalyticreduction (NH₃-SCR) catalyst comprising copper and a zeolite selectedfrom the group of Framework Type consisting of an AEI or a CHA, whereinthe modified LNT stores NO_(x) at temperatures below about 200° C. andreleases the stored NO_(x) at temperatures above about 200° C. and themodified LNT comprises: (a) a first layer comprising a NO_(x) adsorbentcomponent and one or more platinum group metals selected from the groupconsisting of palladium, platinum, and mixtures thereof; and (b) asecond layer comprising a diesel oxidation catalyst zone comprising aplatinum group metal, a zeolite, and optionally an alkaline earth metal;and an NO oxidation zone comprising platinum group metal and a carrier.2. The exhaust system of claim 1, wherein the urea injection systeminjects urea at temperatures above about 180° C.
 3. The exhaust systemof claim 2, wherein the NO_(x) adsorbent component comprises an alkalineearth metal, an alkali metal, a rare-earth metal, and mixtures thereof.4. The exhaust system of claim 3, wherein the alkaline earth metal, analkali metal, a rare-earth metal, and mixtures thereof is supported onan inorganic oxide material.
 5. The exhaust system of claim 4, whereinthe inorganic oxide material is a ceria-containing material or amagnesia-alumina.
 6. The exhaust system of claim 5, wherein themagnesia-alumina is a magnesium-aluminate spinel.
 7. The exhaust systemof claim 1, wherein the NO_(x) adsorbent component comprises barium. 8.The exhaust system of claim 5, wherein the zeolite of the dieseloxidation catalyst zone is selected from the group of Framework Typeconsisting of an AEI, a CHA, a LEV, a BEA, a FAU, and MFI.
 9. Theexhaust system of claim 8, wherein the diesel oxidation catalyst zonefurther comprises an inorganic oxide support comprising an alumina or asilica-doped alumina.
 10. The exhaust system of claim 9, wherein thecarrier of the NO oxidation zone comprises alumina and cerin.
 11. Theexhaust system of claim 10, wherein the NO oxidation zone furthercomprises manganese.
 12. The exhaust system of claim 11 wherein theNH₃-SCR catalyst is an ammonia-selective catalytic reduction filter(NH₃-SCRF).
 13. A method for treating exhaust gas from an internalcombustion engine of a vehicle, comprising: (a) passing the exhaust gasover a modified lean NO_(x) trap (LNT) to remove oxides of nitrogen(NO_(x)) from the exhaust gas at temperatures below about 200° C. andrelease the NO_(x) at temperatures above about 200° C.; (b) injectingurea into the exhaust gas downstream of the modified LNT at temperaturesabove about 180° C.; and (c) passing an exhaust gas containing releasedNO_(x) from the modified LNT and urea over a NH₃-SCR catalyst comprisingcopper and a zeolite selected from the group of Framework Typeconsisting of an AEI or a CHA to convert the NO_(x) to nitrogen.
 14. Themethod of claim 13 further comprising periodically subjecting themodified LNT to a temperature above about 500° C. in a rich air:fuelratio environment to remove sulfur that has accumulated on the modifiedLNT.